Mercury (Hg) is a global pollutant and potent neurotoxin.1 Over the last 50 years over a million people have suffered from mercury poisoning, many of which are newborns that are exposed to it in the womb. The United Nations have recognized Hg as the #1 priority contaminant across the globe and have issued a mandate for the development of new cost effective and environmentally sustainable technologies to limit the impact of Hg on environmental and human health.
The accumulation of mercury (Hg) in rice, a dietary staple for over half of the world's population, is rapidly becoming a global food safety issue. Rice paddies support the anaerobic production of toxic methylmercury that accumulates in plant tissue, however the microbial controls of Hg cycling in anoxic environments remain poorly understood.
Although all chemical species of Hg are toxic, methylmercury (MeHg) is perhaps the most concerning because it bioaccumulates in the tissues of plants2 and animals1. Whereas fish consumption has long been thought of as a primary route of Hg exposure for humans, contaminated rice is becoming an emerging health concern in countries facing Hg pollution issues where rice is a dietary staple2,3.
Rice paddies are often flooded, leading to prolonged periods of anoxia and are rich with nutrients that stimulate the metabolism of iron and sulfate reducers, fermentative bacteria, and methanogenic archaea4. These groups of microbes are directly responsible for Hg methylation and make rice paddies methylation hotspots3,5-9. Similar hotspots can be found in other surface and near subsurface environments such as sediments10 and groundwater11. To mitigate risk, better characterization of pathways controlling availability of inorganic Hg substrate for methylation may be particularly important12,13.
Traditional Hg remediation technologies, and their use, typically varies depending on the site in question. Generally, such techniques may be divided into soil and water treatment strategies. Soil methods have included thermal treatment, soil washing, and soil stabilization. Water treatment methods have included Hg precipitation as a sulfide-bearing mineral, coagulation, filtration by reverse osmosis, adsorption by activated carbon, and ion exchange. Bioremediation (techniques that use living organisms for Hg removal) have been employed in both soil and water environments. Typically, bioremediation techniques have been limited to the use of plants (phytoremediation) or microbes with dedicated Hg detoxification machinery. Both bioremediation approaches have been used to a much lesser degree relative to the aforementioned physical/chemical techniques.
Disadvantages of traditional technologies typically include one or more of: high-energy costs (i.e. thermal treatment and soil washing); secondary waste production through the use of harmful chemicals (i.e. soil washing, soil stabilization, coagulation); and/or low-efficiency Hg removal (i.e. adsorption by activated carbon). Even techniques such as ion exchange have been limited by the fact that Hg does not leave the system, instead being converted to a less mobile chemical species. Costly long-term monitoring efforts are often involved to ensure the contaminant does not leach back into the environment.
Traditionally, bioremediation methods have included: using plants to absorb and/or volatilize Hg; the use of microbes or microbe-derived materials as a biosorbent or to precipitate Hg as a solid; and using Hg-resistant microbes capable of enzymatically converting Hg into a gas. Although phytoremediation is considered to be amongst the most environmentally friendly techniques, it has been limited by the high costs associated with growing and maintaining healthy plants in contaminated environments. The use of phytoremediation is also limited by the cost of properly disposing of plants contaminated with Hg, which creates an issue of secondary waste.
In terms of microbe use, a drawback for using biosorbents and microbially-catalyzed precipitation has been that the waste still needs to be monitored to ensure Hg is not getting back into the environment. Similar to the use of plants, the contaminated biomass still needs to be disposed of properly. While the use of certain Hg resistant bacteria may allow for the conversion of Hg to a gas, a notable drawback is that high levels of Hg, oxygen, and growth substrate have been required to initiate Hg removal, meaning that they are not suited for all environments/applications (i.e. particularly those devoid of oxygen). In cases where microbial strains have been genetically engineered for Hg removal, there are also concerns linked to introducing genetically modified bacteria into the environment that can disrupt ecosystem function.
Alternative, additional, and/or improved methods and processes for the removal of mercury from aqueous solutions is desirable.